Radio frequency (RF) transceivers may use quadrature modulation for higher spectral efficiency. The quadrature signals that are used for modulation and demodulation directly affect the performance of the transceiver and thus it is desirable that the quadrature signals be precise and have a low phase noise. Consequently, these signals may be generated locally at the transceiver.
Generating precise, high-quality, low phase-noise quadrature signals with a phase-shift of π/2 may be very difficult. In some conventional oscillators, two signals having a frequency 2f0 are generated with a phase difference of π. This phase difference may be produced precisely and straightforwardly, since the two signals have exact opposite phases. Two or more digital frequency dividers may be used to convert the two 2f0 signals into two signals of frequency f0, respectively, with a phase difference of π/2.
Unfortunately, digital frequency dividers are high bandwidth devices, and consequently, the quadrature signals at their output may introduce undesired phase noise and other artifacts into the signal ultimately used for modulation and/or demodulation.